The use of triazoles for inhibiting the corrosion of copper and iron alloys in a wide variety of aqueous systems is well known. In industrial cooling water systems, benzotriazole and tolyltriazole are used most often. Tolyltriazole is generally preferred because of its lower cost. Triazoles are film forming materials that provide efficient coverage of metal or metal oxide surfaces in a system thereby providing protection against corrosive elements present in an aqueous system. In addition to the film forming tendency of various azoles, they also precipitate soluble, divalent copper ions. The precipitation of copper ions prevents the transport of copper ions to ferrous surfaces, where galvanic reactions between copper ions and iron atoms leads to pitting corrosion of the ferrous metal.
While the use of azoles for corrosion inhibition is widespread, there are drawbacks to their use, specifically with tolyltriazole. The most important drawbacks are experienced when azoles are used in combination with oxidizing halogen biocides. Oxidizing halogens such as elemental chlorine, bromine, their hypohalous acids, or their alkaline solutions (i.e., solutions of hypochlorite or hypobromite ions) are the most common materials used to control microbiological growth in cooling water systems. When copper or iron alloys that have been previously protected with azoles are exposed to an oxidizing halogen, corrosion protection breaks down. After breakdown, it is difficult to form new protective films in tolyltriazole treated systems that are being halogenated, particularly continuously halogenated. Very high dosages of tolyltriazole are frequently applied in an attempt to improve performance, often with limited success.
The degradation of protection of azole films in the presence of oxidizing halogens is well documented in the literature. For example, U.S. Pat. No. 5,772,919 discloses the use of halo-benzotriazoles as corrosion inhibitors in aqueous systems. The halo-benzotriazoles disclosed in U.S. Pat. No. 5,772,919 were found to be effective in the presence of chlorine.
U.S. Pat. No. 4,642,221 discloses the use of aromatic triazoles such as benzene triazole and derivatives of benzotriazoles such as alkyl-substituted triazoles in combination with an imino compound to control corrosion in aqueous systems.
U.S. Pat. No. 4,184,991 discloses a composition and method of inhibiting corrosion of ferrous metals comprising treatment of aqueous systems with an admixture of a benzotriazole, a tolyltriazole, a substituted benzotriazole or a substitute tolyltriazole with an acrylic or methacrylic acid ester polymer.
U.S. Pat. No. 5,217,686 discloses a corrosion inhibitor which comprises an alkoxybenzotriazole in combination with mercaptobenzothiazole, tolyltriazole, benzotriazole, a substituted benzotriazole and/or 1-phenyl-5-mercaptotetrazole.
U.S. Pat. No. 5,141,675 discloses a corrosion inhibitor comprising a polyphosphate in combination with an azole such as an alkyl or alkoxy benzotriazole, mercaptobenzothiazole, tolyltriazole, benzotriazole, a substituted benzotriazole and/or 1 -phenyl-5-mercaptotetrazole.